1. Field of the Invention
The present invention relates to a photosensitive lithocholate derivative and to a chemically amplified photoresist composition containing the same.
This application is a counterpart of, and claims priority to, Korean Application No. 99-39337, filed Sep. 14, 1999, the contents of which are incorporated herein by reference.
2. Description of the Related Art
As semiconductor devices become highly integrated, photolithography processes used in the fabrication of such devices must be capable of forming ultra-fine patterns. For example, a sub-quarter micron or smaller sized pattern is needed in a semiconductor memory device having a capacity exceeding 1 Gbit. As such, a photolithography technology has beery proposed which employs an argon fluoride (ArF) excimer laser as a new type of light source. This is because the ArF laser exhibits a wavelength (193 nm) which is shorter than the wavelength (248 nm) of a conventional krypton fluoride KrF excimer laser. Therefore, a demand has arisen for chemically amplified photoresist polymers and photoresist compositions which are suitable for use with the ArF excimer laser.
In general, a chemically amplified photoresist composition for an ArF excimer laser should exhibit the following characteristics: (1) transparency at a wavelength of 193 nm; (2) high resistance to dry etching; (3) good adhesion to underlying (and overlying) film materials; and (4) easily capable of being developed using developing solutions which are in widespread use in the manufacture of semiconductor devices.
However, a terpolymer comprising methylmethacrylate, t-butyl methacrylate and methacrylic acid monomers, which is a widely known chemically amplified photoresist polymer for the ArF excimer laser, does not exhibit all of the above-mentioned characteristics. In particular, the terpolymer has a very low resistance to dry etching, and a low adhesion to underlying film materials.
Recently, attempts have been made to increase the etching resistance of a photosensitive polymer for the ArF excimer laser by introducing alicyclic compounds, for example, isobornyl, adamantyl or tricyclodecanyl group, into the backbone of the polymers. However, these polymers also have several disadvantages. For example, their etching resistance is still not acceptable and their adhesion characteristics to underlying films are still poor, which results in lifting of photoresist patterns.
U.S. Pat. No. 5,786,131 discloses mono-, di- ortri-protected hydroxy androstane-17-alkylcarboxylate such as t-butyl-3-acetyllithocholate, t-butyl 3-trifluoroacetyllithocholate or t-butyl lithocholate as an additives for increasing the etching resistance of a resist composition having poor etching resistance. While the lithocholate derivatives disclosed in this patent can be used as an additive for increasing the etching resistance of the resist composition, they cannot by themselves be used as a main component of the resist composition since they are brittle and exhibit poor wettability against a developing solution.
It is an objective of the present invention to provide a photosensitive compound which maintains transparency even when exposed to a short-wavelength light source of 193 nm or below, and which has improved adhesion to an underlying film or substrate, improved wettability to a developing solution and improved resistance to dry etching.
It is another objective of the present invention to provide a chemically amplified photoresist composition containing the photosensitive compound.
According to one aspect of the present invention, there is provided a photosensitive lithocholate derivative having a carboxylic acid protected with a protective group capable of being deprotected with an acid and having a hydroxy at position No. 3 substituted with a hydrophillic atliphatic compound or a hydrophillic alicyclic compound selected from the group consisting of alken glycol, poly(alken glycol), alken glycol aliphatic hydrocarbon ether and alken glycol alicyclic hydrocarbon ether, poly(alken glycol) aliphatic hydrocarbon ether and poly(alken glycol) alicyclic hydrocarbon ether.
According to another aspect of the present invention, there is provided a chemically amplified photoresist composition including a first photosensitive lithocholate derivative, a second photosensitive lithocholate derivative, or a mixture thereof, and a photoacid generator contained in an amount of 1 to 15% by weight based on the total weight of the photosensitive lithocholate, wherein the first photosensitive lithocholate derivative has a carboxylic acid protected with a protective group capable of being deprotected with an acid and having a hydroxy at position No. 3 substituted with a hydrophillic aliphatic compound or a hydrophillic alicyclic compound selected from the group consisting of alken glycol, poly(alken glycol), alken glycol aliphatic hydrocarbon ether and alken glycol alicyclic hydrocarbon ether, poly(alken glycol) aliphatic hydrocarbon ether and poly(alken glycol) alicyclic hydrocarbon ether, and the second photosensitive lithocholate derivative has a carboxylic acid protected with a protective group capable of being deprotected with anacid and has a hydroxy at position No. 3 substituted with a hydrophillic alicyclic compound selected from the group consisting of alken glycol lithocholic ether, alken glycol lithocholate ether, poly(alken glycol) lithocholic ether and poly(alken glycol) lithocholate ether.
The chemically amplified photoresist composition may further include 0.01 to 2.0% by weight of an organic base based on the total weight of the photosensitive compound.
The photosensitive compound according to the present invention maintains transparency even when exposed to a short-wavelength light source, and exhibits improved adhesion to an underlying film or substrate, improved wettability to a developing solution, and improved resistance to dry etching.
A photosensitive compound and a chemically amplified photoresist composition containing the same according to the present invention will now be described. Also, a preferred photolithography process using the chemically amplified photoresist composition will also be described. This invention may, however, be embodied in many different forms, and these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.
A photosensitive compound according to the present invention is a photosensitive lithocholate derivative having a carboxylic acid protected with a protective group capable of being deprotected with an acid and having a hydroxy at position No. 3 substituted with hydrophillic aliphatic compound or hydrophillic alicyclic compound.
The protective group causes the lithocholate derivative to exhibit insolubility to a developing solution when it is bonded thereto. However, if the protective group is deprotected, that is, decomposed by an acid generated during exposure, a lithocholic acid derivative is generated, thereby exhibiting a greater solubility to a developing solution.
Thus, in consideration of a difference in the solubility of the photosensitive compound before and after exposure, the protective group is preferably t-butyl, 2-methyladamantyl, tetrahydropyranyJ or tetrahyd-rofuranyl.
The photosensitive lithocholate derivative according to a first embodiment of the present invention has a hydroxy at position No. 3 substituted with a hydrophillic aliphatic compound or a hydrophillic alicyclic compound selected from the group consisting of alken glycol, poly(alken glycol), alken glycol aliphatic hydrocarbon ether, alken glycol alicyclic hydrocarbon ether, poly(alken glycol) aliphatic hydrocarbon ether and poly(alken glycol) alicyclic hydrocarbon ether.
Here, the alken glycol is preferably C2 to C6 alken glycol, more preferably ethylene glycol or propylene glycol. The number of alken glycols in poly(alken glycol) is determined in consideration of the molecular weight and polarity of the photosensitive lithocholate derivative. Preferably, poly(alken glycol) consists of 2 to 50 alken glycols.
The aliphatic hydrocarbon and alicyclic hydrocarbon substituted for the hydroxy of alken glycol or poly(alken glycol) to form ether is a C1 to C20 hydrocarbon.
The molecular weight and polarity of the photosensitive lithocholate derivative can be adjusted by varying the kind of substituent at position No. 3.
A photosensitive compound according to a first embodiment of the present invention can be represented by the formula (1): 
wherein R1 is a protective group, R2 is hydrogen, a C1 to C20 aliphatic hydrocarbon or a C1 to C20 alicyclic hydrocarbon, n is an integer of 1 to 5, and m is an integer of 1 to 50.
A photosensitive lithocholate derivative according to a second embodiment of the present invention has a hydroxy at a position of No. 3 substituted with a hydrophillic alicyclic compound selected from the group consisting of alken glycol lithocholic acid ether, alken glycol lithocholate ether, poly(alken glycol) lithocholic ether and poly(alken glycol) lithocholate ether.
Here, like in the first embodiment, the alken glycol is preferably C2 to C6 alken glycol, more preferably ethylene glycol or propylene glycol. The poly(alken glycol) consists of 2 to 50 alken glycols.
The lithocholate substituted for the alken glycol or poly(alken glycol) to form the ether compound preferably has a protective group which can be deprotected by an acid or a dissolution inhibitor group substituted for a carboxyl group.
The protective group is preferably t-butyl, 2-methyladamantyl, tetrahydropyranyl or tetrahydrofuranyl.
The dissolution inhibitor group is a hydrophobic group which is not decomposed by an acid generated during exposure, and is introduced for preventing an unexposed region from being developed due to a large amount of hydrophillic functional group, e.g., a carboxyl group, in the photosensitive compound in the case of using a general developing solution. Examples of the dissolution inhibitor group include a C1 to C20 aliphatic hydrocarbon or a C1 to C20 alicyclic hydrocarbon.
A photosensitive compound according to a second embodiment of the present invention can be represented by the formula (2): 
wherein R1 is a protective group, R3 is hydrogen, a protective group or a dissolution inhibitor group, n is an integer of 1 to 5, and m is an integer of 1 to 50.
The photosensitive compound according to the present invention is transparent even when exposed to a short-wavelength light source of 193 nm or below. Also, since the photosensitive compound has an alicyclic structure, the etching resistance thereof is large. Further, since a hydrophillic aliphatic compound or a hydrophillic alicyclic compound is substituted for a hydroxy at position No. 3, the compound exhibits good adhesion to an underlying film or substrate and excellent wettability to a developing solution. Also, the molecular weight and polarity of the photosensitive lithocholate derivative can be easily adjusted by varying the kind of substituent introduced to a hydroxy.
A chemically amplified photoresist composition of the present invention includes the above-described photosensitive compound and a photoacid generator.
The photoacid generator is preferably contained in an amount of 1 to 15% by weight based on the total weight of the photosensitive compound.
The photoacid generator is preferably a substance that has a high thermal stability. Therefore, suitable photoacid generators include triarylsulfonium salts, diaryliodonium salts, sulfonates or N-hydroxysuccinimide triflates.
Examples of photoacid generators include triphenylsulfonium triflate, triphenylsulfonium antimonate, diphenyliodonium triflate, diphenyliodonium antimonate, methoxydiphenyliodonium triflate, di-t-butyidiphenyliodonium triflate, 2,6-dinitro benzyl sulfonate or pyrogallol tris(alkyl-sulfonates), pyrogalloltris(alkylsulfonates)), norbornene-dicarboximide triflate, triphenylsulfonium nonaflate, diphenyliodonium nonaflate, methoxydiphenyliodonium nonaflate, di-t-butyl diphenyliodonium nonaflate, N-hydroxysuccinimide nonaflates, norbornene dicarboximide nonaflate, triphenylsulfonium perfluorooctanesulfonates, diphenyliodonium perfluorooctanesulfonates, methoxydiphenyliodonium perfluorooctane sulfonates, di-t-butyidiphenyliodonium triflate, N-hydroxysuccinimide perfluorooctanesulfonates, or norbornene dicarboximide perfluorooctanesulfonates.
Preferably, the chemically amplified photoresist composition of the present invention further includes 0.01 to 2.0% by weight of organic base based on the total weight of the photosensitive compound. Suitable organic bases include triethylamine, triisobutylamine, triisooctylamine, diethanolamine, triethanolamine or a mixture thereof. The organic base is added for preventing a pattern from being deformed due to the acidolysis of photoresist composition forming unexposed regions, after exposure, which results from diffusion of the acid generated at the exposed regions into the unexposed regions.
Also, the chemically amplified photoresist composition according to the present invention preferably further includes 30 to 200 ppm of an organic or base surfactant. The surfactant functions to allow the photoresist composition to be uniformly coated on a substrate.
Synthesis of Tosylated Hydrophilic Compound
A hydrophilic aliphatic compound or a hydrophilic alicyclic compound (I) is reacted with p-toluene sulfonyl chloride (TsCI) (II) to tosylate the same, as expressed by the following reaction scheme (1): 
wherein R2 is hydrogen or a C1 to C20 aliphatic hydrocarbon group or a C1 to C20 alicyclic hydrocarbon group, n is an integer from 1 to 5, and m is an integer from 1 to 50.
Synthesis of Hydrophilic Compound Substituted Lithocholate
A tosylated hydrophilic compound (III) is reacted with a lithocholate (IV) as expressed by the following reaction scheme (2): 
wherein R1 is a protective group. Then, a hydrophilic compound substituted lithocholate is formed by SN2 reaction.
Synthesis of Tosylated Hydrophilic Compound
Alken glycol or poly(alken glycol) (V) is reacted with TsCI (II) to prepare a compound (VI) having two hydroxy groups tosylated, as expressed by the following reaction scheme (3): 
Synthesis of Hydrophilic Alicyclic Compound Substituted Lithocholate
Tosylated alken glycol or tosylated poly(alken glycol) (VI) is reacted with first lithocholate (VII) and second lithocholate (VIII) to prepare hydrophilic alicyclic compound substituted lithocholate by SN2 reaction, as expressed by the following reaction scheme (4): 
wherein R1 is a protective group, R3 is hydrogen, a protective group or a dissolution inhibitor group, n is an integer from 1 to 5, and m is an integer from 1 to 50. Then, a hydrophilic compound substituted lithocholate is formed by SN2 reaction.
The chemically amplified photoresist composition according to the present invention is prepared by dissolving the photosensitive compound prepared in the above-described manner and a photoacid generator, in an appropriate solvent, for example, propylene glycol monomethyl ether acetate (PGMEA), and mixing the same. Here, the photoacid generator is mixed in an amount of 1 to 15% by weight based on the weight of the polymer. Also, it is preferable to complete the photoresist composition by further dissolving 0.01 to 2.0% by weight of an organic base based on the weight of the polymer. Also, 30 to 200 ppm of a surfactant is preferably further included in the composition.
The chemically amplified photoresist composition prepared in the above-described manner can be used for a general photolithography process, and is particularly suitable for forming a fine pattern which satisfies a design rule of 0.20 xcexcm or smaller using an ArF excimer laser as an exposure light source.
First, the photoresist composition is coated on a substrate where a patterning object material layer is formed, to form a photoresist layer having a predetermined thickness, preferably 0.2 to 2 xcexcm. Subsequently, pre-baking is carried out on the photoresist layer. The pre-baking step is performed at a temperature of 70 to 160 xc2x0 C. for 30 to 360 seconds. After the pre-baking step, the photoresist layer is exposed using a mask having a predetermined pattern, using an exposure light source having a wavelength of 248 nm or less, preferably an ArF excimer laser having a wavelength of 193 nm. Acid is generated from the photoacid generator contained in the photoresist layer by exposure. The photosensitive compound is acidolyzed by the catalytic action of the thus-generated acid to form a lot of carboxy groups, as expressed by reaction scheme 5. As a result, a large amount of hydrophilic groups, e.g., carboxy groups, are produced in the exposed region of the photoresist layer. Thus, a noticeable difference in the polarity of the photoresist layer is created between an exposed region and an unexposed region. That is, contrast is noticeably increased. 
The parentheses enclosing R3 denote that R3 is not acidolyzed but is retained since it is a dissolution inhibitor group.
After exposure, the photoresist layer is thermally treated for a short time before development, which is referred to as a post-exposure-thermal treatment. The post-exposure-thermal treatment is performed for the purpose of increasing contrast by further activating acidolysis of exposed regions by the acidic catalyst, that is, for the purpose of increasing contrast by acidolyzing a protective group contained in the photosensitive compound in the exposed region.
Next, development is performed using an appropriate developing solution to complete a photoresist pattern. Here, the developing solution used is a developing solution for general development processes, for example, 2.38% by weight of tetramethylammonium hydroxide (TMAH). Since the photosensitive compound according to the present invention has a polarity, it exhibits excellent wettability against a developing solution. Thus, development is facilitated by using the conventional developing solution.
After forming the photoresist pattern, a patterning object layer is etched to form a desired pattern. The photoresist pattern of the present invention is formed of a photoresist compound having several alicyclic structures, and thus the etching resistance thereof is large. Therefore, a pattern having a good profile, that is, having a precise critical dimension, can be formed.